Reflective polarizer assembly

ABSTRACT

An assembly that includes a first reflective polarizer substantially reflecting light having a first polarization state and substantially transmitting light having a second polarization state, a polarization rotating layer or depolarizing layer (or both) positioned to receive light passing through the first reflective polarizer, and a second reflective polarizer positioned to receive light passing through the polarization rotating layer or depolarizing layer, the second reflective polarizer substantially reflecting light having a third polarization state back through the polarization rotating layer or depolarizing and substantially transmitting light having a fourth polarization state. Articles containing the assembly can be formed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 11/427,202, filedJun. 28, 2006 which is a continuation of U.S. Ser. No. 10/919,707, filedAug. 17, 2004, now issued as U.S. Pat. No. 7,084,938, which is acontinuation of U.S. Ser. No. 09/968,817, filed Oct. 1, 2001, now issuedas U.S. Pat. No. 6,985,291 on Jan. 10, 2006, the disclosure of which isherein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to an assembly of optical elements. Inanother aspect, the invention relates to a method of making the same.

BACKGROUND OF THE INVENTION

The physical downsizing of microprocessor based technologies has led toportable personal computers, pocket secretaries, wireless phones andpagers. All of these devices, and also other devices such as clocks,watches, calculators, etc., have the common need for a low powerconsumption data display screen to extend the useful working timebetween battery replacements or battery charges. The common LiquidCrystal Display (LCD) is often used as the display for such devices.LCDs can be classified based upon the source of illumination. Reflectivedisplays are illuminated by ambient light that enters the display fromthe front. In applications where the intensity of ambient light isinsufficient for viewing, supplemental lighting, such as a backlightassembly, is used to illuminate the display. Some electronic displayshave been designed to use ambient light when available and backlightingonly when necessary. This dual function of reflection and transmissionleads to the designation, “transflective”.

A liquid crystal display (LCD) illustrates another example of the use ofpolarized light. FIGS. 1A and 1B schematically illustrate one example ofa simple TN (twisted nematic) LCD device with E-mode transmission andnormally white (NW) operation using a backlight. It will be understoodthat there are a variety of other LCD types and other modes ofoperation, as well as displays that use ambient light or a combinationof a backlight and ambient light. The inventions discussed herein can bereadily applied to these display types and modes of operation.

The LCD 50 of FIGS. 1A and 1B includes a liquid crystal (LC) cell 52, apolarizer 54, an analyzer 56, and a backlight 58. The arrows 55, 57 onthe polarizer 54 and analyzer 56, respectively, indicate thepolarization of light that is transmitted through that component. Arrows51, 53 indicate the plane of polarization of linearly polarized light,respectively entering and exiting the LC cell 52. Additionally, theplane of the LC cell 52 containing arrows 51, 53 generally includestransparent electrodes. Light from the backlight 58 is linearlypolarized by the polarizer 54. In the embodiment illustrated in FIG. 1A,in the absence of an electric potential applied across the LC cell, thedirector substantially lies in the plane of the display twistinguniformly through 90° along its depth. The polarized light istransmitted through the LC cell 52 where the polarization ideallyrotates by 90°, with the director of the liquid crystals indicated bythe arrows 51, 53. This light can then be transmitted through theanalyzer 56.

An electric potential can be applied at electrodes (not shown) proximateto opposing ends of the LC cell 52, setting up an electric field withinthe LC cell. In the case where the LC material has a positive dielectricanisotropy, the director substantially aligns in the direction of theelectric field lines, provided sufficient potential is applied acrossthe electrodes. The director at the center of the cell is orientedperpendicular to the plane of the display in this case. The linearlypolarized light entering the cell is no longer rotated through the 90°required for transmission through the analyzer. In the embodimentillustrated in FIG. 1B, the plane of polarization of the polarized lightas it exits LC cell 52 (designated by arrow 53′) is unchanged from itsoriginal orientation (designated by arrow 51). Hence, the light exitingthe LC cell 52 is not transmitted through the analyzer 56, because thelight exiting the LC cell has the wrong polarization. One method ofobtaining a gray scale includes only applying sufficient electricpotential to partially orient the director of the liquid crystalsbetween the two illustrated configurations. In addition, it will berecognized that a color cell can be formed by, for example, using colorfilters.

Typically, the polarizer 54 and analyzer 56 are constructed usingabsorbing sheet polarizers because these polarizers have good extinctionof light having the unwanted polarization. This, however, results insubstantial loss of light because the backlight generally emitsunpolarized light. Light of the unwanted polarization is absorbed by thepolarizer. As an alternative configuration (illustrated in FIG. 1C), areflective polarizer 60 is placed between the polarizer 54 and thebacklight 58. The reflective polarizer reflects light with the unwantedpolarization back towards the backlight. The reflected light can berecycled using a reflector 62 behind the backlight where a substantialportion of the reflected light can be reused.

SUMMARY OF THE INVENTION

Generally, the present invention relates to a reflecive polarizerassembly, articles containing reflective polarizer assemblies, andmethods of using and making the same.

One embodiment is an assembly that includes a first reflective polarizersubstantially reflecting light having a first polarization state andsubstantially transmitting light having a second polarization state, apolarization rotating layer positioned to receive light passing throughthe first reflective polarizer, and a second reflective polarizerpositioned to receive light passing through the polarization rotatinglayer, the second reflective polarizer substantially reflecting lighthaving a third polarization state back through the polarization rotatinglayer and substantially transmitting light having a fourth polarizationstate. Optionally, the polarization rotating layer is positioned toreceive light reflected by the second reflective polarizer. The assemblycan also include an absorbing polarizer, a depolarizing layer, or both.The assembly can be used in conjunction with a backlight system.

Another embodiment is another assembly that includes a first reflectivepolarizer substantially reflecting light having a first polarizationstate and substantially transmitting light having a second polarizationstate, a depolarizing layer positioned to receive light passing throughthe first reflective polarizer, the depolarizing layer converting aportion of incident light into an orthogonal polarization state, and asecond reflective polarizer positioned to receive light passing throughthe depolarizing layer, the second reflective polarizer substantiallyreflecting light having a third polarization state back through thedepolarizing layer and substantially transmitting light having a fourthpolarization state. Optionally, the assembly can include a polarizationrotating layer. The assembly can also include an absorbing polarizer.The assembly can be used in conjunction with a backlight system.

Yet another embodiment is a display that includes a liquid crystal cellthat is configured and arranged to operate using polarized light; alight source; and one of the previously described assemblies disposedbetween the liquid crystal display cell and the light source.

Another embodiment is a method of polarizing light. The light isdirected at a first reflective polarizer. The first reflective polarizersubstantially reflecting light having a first polarization state andsubstantially transmitting light having a second polarization state. Thelight having a first polarization state is rotated. A second reflectivepolarizer receives the light that is rotated. The light reflected by thesecond reflective polarizer is rotated and provided to the firstreflective polarizer.

The above summary of the present invention is not intended to describeeach disclosed embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1A is a schematic perspective view of one embodiment of a TN LCD;

FIG. 1B is a schematic perspective view of the LCD of FIG. 1A in which apotential has been applied across the LC cell of the LCD;

FIG. 1C is a schematic perspective view of a second embodiment of anLCD;

FIG. 2 is a schematic cross-sectional view of one embodiment of anassembly according to the invention;

FIG. 3 is a schematic cross-sectional view of a second embodiment of anassembly according to the invention;

FIG. 4 is a schematic cross-sectional view of a third embodiment of anassembly according to the invention;

FIG. 5 is a schematic cross-sectional view of a fourth embodiment of anassembly according to the invention;

FIG. 6 is a schematic cross-sectional view of a fifth embodiment of anassembly according to the invention;

FIG. 7 is a schematic cross-sectional view of a sixth embodiment of anassembly according to the invention;

FIG. 8 is a schematic perspective view of one embodiment of an LCD,according to the invention; and

FIG. 9 is a schematic perspective view of another embodiment of an LCD,according to the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

In one implementation, the present invention is directed to a method ofembodying a non-inverting transflector in a roll-to-roll laminationprocess. Articles of the present invention generally include reflectivepolarizers between which is located a polarization rotator, adepolarizing layer, or both. The polarization rotator can be, forexample, a polarizer, a compensation film, a Brewster-type polarizingdevice, or a polarizing lightguide. One embodiment of the inventioninvolves linear reflecting polarizer films with parallel polarizationaxes, between which is located a polarization rotator, a depolarizinglayer, or both.

The transflective displays described can also use the advantageousproperties of multilayer optical films as reflective polarizers. Theadvantages, characteristics and manufacturing of such films aredescribed, for example, in U.S. Pat. Nos. 5,882,774 and 5,965,247, bothof which are herein incorporated by reference. The multilayer opticalfilm is useful, for example, as mirrors or polarizers.

Multilayer optical films as reflective or absorbing polarizers as usedin conjunction with the present invention exhibit relatively lowabsorption of incident light, as well as high reflectivity for off-axisas well as normal light rays. These properties generally hold whetherthe films are used for reflection or absorption of polarized light. Theunique properties and advantages of the multi-layer optical filmprovides an opportunity to design highly efficient transflectivedisplays. Transflective displays are described, for example, in U.S.Pat. No. 6,124,971, herein incorporated by reference.

FIG. 2 is a schematic cross-sectional view of one embodiment of atransflective assembly according to the invention. The transflectiveassembly 100 typically includes an absorbing polarizer 102, a reflectivepolarizer 104, a polarization rotator 106, and a second reflectivepolarizer 108. The transflective assembly may be coupled to abacklighting system 110 with back reflector. In some embodiments, thetransmission axes of the absorbing polarizer 102, reflective polarizer104, and second reflective polarizer 108 are substantially parallel;however, this is not necessary to the invention. The transmission axisis the axis that permits the maximum transmission of the light. Asdefined herein, substantially parallel transmission axes are within 5°of each other. Optionally, a polarization rotator can be providedbetween the absorbing polarizer 102 and the reflective polarizer 104,particularly if the two polarizers have different polarization axes.

FIG. 3 is a schematic cross-sectional view of another embodiment of atransflective assembly according to the invention. The transflectiveassembly 100 includes an absorbing polarizer 102, a reflective polarizer104, a depolarizing layer 112, and a second reflective polarizer 108.The transflective assembly may be coupled to a backlighting system 110with back reflector. The backlighting system may include any lightsource suitable for the particular application of the transflectiveassembly 100. Optionally, a polarization rotator can be provided betweenthe absorbing polarizer 102 and the reflective polarizer 104,particularly if the two polarizers have different polarization axes.

FIG. 4 is a schematic cross-sectional view of another embodiment of atransflective assembly according to the invention. The transflectiveassembly 100 includes an absorbing polarizer 102, a reflective polarizer104, a depolarizing layer 112, a polarization rotator 106, and a secondreflective polarizer 108. The transflective assembly may be coupled to abacklighting system 110 with back reflector. The backlighting system mayinclude any light source suitable for the particular application of thetransflective assembly 1 00. Optionally, a polarization rotator can beprovided between the absorbing polarizer 102 and the reflectivepolarizer 104, particularly if the two polarizers have differentpolarization axes.

In some embodiments, in reflective mode, light having the appropriatepolarization properties to pass through the absorbing polarizer 102 willalso pass through the reflective polarizer 104. The polarization rotator106 changes the overall direction of polarization of the transmittedlight so that a portion of the light will be reflected by reflectingpolarizer 108. Preferably, the polarization rotator 106 rotates thepolarization of light by 45° or more. Light that is not reflected willenter the backlighting system 110, from which it may reflect andeventually re-enter the transflecting assembly 100. Light reflected fromreflecting polarizer 108 back towards the polarization rotator 106 willhave its polarization modified a second time, and some portion will passthrough reflective polarizer 104 and return to the viewer. If the lightis reflected from reflective polarizer 104, the process will continue.In other embodiments, light passing through the depolarizing layer 112has its polarization reduced and a portion of the light will bereflected by reflecting polarizer 108. Light that is not reflected willenter the backlighting system 110, from which it may reflect andeventually re-enter the transflecting assembly 100. Light reflected fromreflecting polarizer 108 back towards the depolarizing layer 106 willhave its polarization reduced a second time, and some portion will passthrough reflective polarizer 104 and return to the viewer. If the lightis reflected from reflective polarizer 104, the process will continue.

FIG. 5 is a schematic cross-sectional view of yet another embodiment ofa transflective assembly according to the invention wherein an absorbingpolarizer 102 is not provided. The transflective assembly 100 includes areflective polarizer 104, a polarization rotator 106, and a secondreflective polarizer 108.

FIG. 6 is a schematic cross-sectional view of yet another embodiment ofa transflective assembly according to the invention wherein an absorbingpolarizer 102 is not provided. The transflective assembly 100 includes areflective polarizer 104, a depolarizing layer 112, and a secondreflective polarizer 108.

One method of manufacturing embodiments of the transflective assembly100 is through a roll to roll process. The process can be used to createa reflective polarizer/absorbing polarizer combination with parallelpass axes. The processes can be performed using roll-to-roll techniques.As an example, a film of a polarizing or polarization altering element,such as a reflective polarizer or absorbing polarizer or both, isunwound from a roll. An alignment layer is formed on the film by, forexample, coating or otherwise disposing a photoalignment material ontothe film, typically with a solvent or dispersant. The coating can beperformed in one or more coating steps. The photoalignment material isoptionally dried to at least partially (preferably, substantially orfully) remove the solvent or dispersant. The photoalignment material iscured using ultraviolet light polarized along the desired alignmentdirection to produce the alignment layer. The curing can be performedprior to or subsequent to the disposition of a liquid crystal materialon the alignment layer, as described below. Alternatives to coating andaligning a photoalignment material to form the alignment layer include,for example, using a polarizing or polarization altering element with analigned surface; coating or otherwise disposing an alignment layer andthen stretching, rubbing, or otherwise mechanically orienting thealignment layer; or sputtering material on the film at an oblique angleto form the alignment layer.

The alignment layer is then coated with the liquid crystal material,typically with a solvent or dispersant. The coating can be performed inone or more coating steps. The liquid crystal material is optionallydried to at least partially (preferably, substantially or fully) removethe solvent or dispersant. In an alternative process, the alignmentlayer and liquid crystal material can be deposited simultaneously, forexample, by coextrusion, on the polarizing or polarization alteringelement.

In addition, a substrate film, such as a TAC film or other optical film,such as a polarizer (e.g., a reflective polarizer or absorbingpolarizer) or compensation film, is unwound and an alignment layer isformed on this film in the same manner. This can be done simultaneously,prior to, or subsequent to the formation of an alignment layer andliquid crystal material on the polarizing or polarization alteringelement. In one alternative embodiment, a liquid crystal material isalso coated onto the substrate film/alignment layer construction. In yetanother alternative embodiment, liquid crystal material is coated orotherwise disposed on the substrate film/alignment layer constructionand not on the polarizing or polarization altering element/alignmentlayer construction.

The coated polarizing or polarization altering element film and thesubstrate film are then brought together (e.g., laminated) so that theliquid crystal material is between the two films. The liquid crystalmaterial is cured using photoactivated, thermal, or e-beam curing toform the polarization rotator. Any photoactivated or e-beam curing istypically done through the substrate film. The final combination is thenwound onto a roll. Preferably, the curing of the liquid crystal materialcouples the two film constructions together.

In another example, a film of a polarizing or polarization alteringelement, such as a reflective polarizer, is unwound from a roll. Analignment layer is formed on the film by, for example, coating aphotoalignment material onto the film, typically with a solvent ordispersant. The coating can be performed in one or more coating steps.The photoalignment material is optionally dried to at least partially(preferably, substantially or fully) remove the solvent or dispersant.The photoalignment material is cured using ultraviolet light polarizedalong the desired alignment direction to produce the alignment layer.Alternatively, any of the other methods described in the previousexample can be used.

The alignment layer is coated with the liquid crystal material,typically with a solvent or dispersant. The coating can be performed inone or more coating steps. The liquid crystal material is optionallydried to at least partially (preferably, substantially or fully) removethe solvent or dispersant. In an alternative process, the alignmentlayer and liquid crystal material can be deposited simultaneously, forexample, by coextrusion, on the polarizing or polarization alteringelement.

Optionally, a second alignment layer is coated or otherwise disposedonto the liquid crystal material, typically with a solvent ordispersant. A second alignment layer preferably may not be needed if thedesired twist angle or retardation can be provided by the liquid crystalmaterial. If the second alignment layer is used, the second alignmentlayer is optionally dried to at least partially (preferably,substantially or fully) remove the solvent or dispersant. In oneembodiment, the second alignment layer includes a photoalignmentmaterial that is cured using ultraviolet light polarized along thedesired alignment direction. In other embodiments, the second alignmentlayer is formed by, for example, disposing a polarizing or polarizationaltering element with an aligned surface on the liquid crystal material;coating or otherwise disposing a second alignment layer on the liquidcrystal material and then stretching, rubbing, or otherwise mechanicallyorienting the second alignment layer; or sputtering material on theliquid crystal material at an oblique angle to form the second alignmentlayer.

The liquid crystal material is cured using photoactivated, thermal, ore-beam curing. Any photoactivated or e-beam curing is typically donethrough the second alignment layer to form the polarization rotator.This curing can occur simultaneously with the second alignment layer (oreven with the first alignment layer or both alignment layers) orsubsequent to the curing of the second alignment layer. The finalcombination is then wound onto a roll.

FIG. 7 is a schematic cross-sectional view of another embodiment of atransflective assembly according to the invention. In an embodiment ofthe transflective assembly 100, the first reflective polarizer 104 andsecond reflective polarizer 108 have non-parallel transmission axes,particularly if the polarizers are not of identical construction. Apolarization rotator 200 is sandwiched between absorbing polarizer 102and reflective polarizer 104 if the absorbing polarizer 102 has atransmission axis not parallel with that of reflective polarizer 104. Insome embodiments, absorbing polarizer 102 and polarization rotator 200are constructed as a single film and laminated to the display, and therest of the transflective assembly 100 is assembled separately. Asdiscussed above, in other embodiments an absorbing polarizer 102 is notused.

The polarization rotator is typically formed using a birefringentmaterial. Examples of suitable birefringent materials include orientedpolymer films, laminated structures of oriented polymer films, and bothorganic and inorganic multilayer birefringent coatings. Other examplesinclude any liquid crystal material that has a director that can becontrolled. A nematic liquid crystal is generally composed of rodlikemolecules with their long axes aligned approximately parallel to oneanother. At any point in the medium one can define a vector to representthe preferred orientation in the immediate neighborhood of the point.This vector is commonly called the director. Suitable liquid crystal(LC) materials include, for example, lyotropic, nematic, and cholestericliquid crystal materials. Examples include E7, BL036, 5CB, and RM257from Merck; C6M, 76, 296, 495, and 716, from Koninklijke PhilipsElectronics N.V. (Amsterdam, the Netherlands); Paliocolor LC242 andPaliocolor CM649from BASF AG (Ludwigshafen, Germany); and LCP-CB483 fromVantico AG (Luxembourg). Additional examples of suitable materialsinclude those described in U.S. Pat. Nos. 5,793,455, 5,978,055, and5,206,752, all of which are incorporated herein by reference. The LCmaterials can be polymeric or monomeric materials. Suitable monomericmaterials also include those materials that can be reacted to formpolymeric liquid crystal materials.

For some embodiments, a twisted nematic LC structure is preferred. Inthese embodiments, the director exhibits a uniform helical twist aboutthe normal to the surface of the polarization rotator. The twist angleand initial orientation can be selected by the use of one or moreoptional alignment layers.

In another embodiment, the axis about which the local director of a LCstructure twists or rotates is not normal to the surface of thesubstrate upon which the LC material is disposed. In this embodiment,the nematic director lies out of the plane of the polarizer orpolarization-altering element. With respect to the surface of thesubstrate, the angle of the axis, at which the local director lies orabout which the local director twists, is defined as the pretilt angle,α. The pitch can be constant or can vary (e.g., increase or decrease)along the axis. The twist angle and orientation can be selected by theuse of one or more optional alignment layers.

At least some liquid crystal materials, such as chiral nematic (e.g.,cholesteric) liquid crystals, include a chiral component which resultsin the formation of a structure where the director of the liquid crystalmaterial naturally rotates about an axis perpendicular to the director.The pitch of the chiral nematic liquid crystal corresponds to thethickness of material needed to achieve a 360° rotation of the director.At least some achiral nematic liquid crystals can be made chiral by theaddition of a chiral compound. The pitch of the material can be modifiedby changing the ratio of chiral to achiral components.

A uniaxial birefringent material, such as a nematic liquid crystal, ischaracterized by two principal refractive indices, n_(o) and n_(e). Theordinary refractive index, n_(o), influences that component of lightwhose electric field polarization vector is perpendicular to the opticalsymmetry axis of the birefringent medium. The extraordinary index,n_(e), influences that component of light whose electric fieldpolarization vector is parallel to the optical symmetry axis of thebirefringent medium (for example, parallel to the director in the caseof a nematic LC material with positive dielectric anisotropy).

The birefringence, An, of the medium can be defined in terms of n_(o)and n_(e):Δn=n _(e) −n _(o).Polarized light incident on a birefringent medium will propagate as anordinary ray component and an extraordinary ray component. The phasevelocity of each component will differ, as each experiences a differentindex of refraction. The total change in phase, or retardation, of thelight depends upon the birefringence and the thickness of the medium.

One embodiment of a suitable polarization rotator corresponds to a layerhaving the thickness of a half wave retarder and an optical axis that isset off from the plane of polarization of incident linearly polarizedlight by an azimuthal angle, φ. The optical axis of the polarizationrotator is in a plane parallel to the “extraordinary” ray andperpendicular to the “ordinary” ray. The half wave retarder rotates thepolarization of the incident linearly polarized light by 2φ. Forexample, a 45° polarization rotator has an optical axis that is set offfrom the polarization direction of incident linearly polarized light by22.5°. The term “half wave retarder” signifies that the polarizationrotator has a thickness, d, with Δnd=(2m+1)λ/2, where λis the wavelengthof light, and m is an integer, 0, 1, 2, . . . . For other wavelengths oflight, the polarization rotator may provide different rotational values.This embodiment functions as a perfect rotator only for wavelengths thatsatisfy the aforementioned requirement.

As yet another example, a polarization rotator can be a formed using aliquid crystal material whose director rotates along the thickness axisof the polarization rotator by a twist angle, Φ, which is much smallerthan a phase retardation, Γ, of the polarization rotator. The phaseretardation is given by:Γ=2πΔnd/λ.When Φ<<Γ for a particular wavelength or wavelength range of light,linearly polarized light incident at one side of the polarizationrotator will emerge rotated by the same amount as the twist angle, Φ,for that wavelength of light. This effect can be achieved when thepolarization rotator includes liquid crystal material having a twistednematic structure. A twisted nematic structure can be achieved usingchiral nematic liquid crystal material or using optional alignmentlayers on opposing sides of the polarization rotator (as illustrated,for example, in FIG. 3) where the alignment between the two layersdiffers by the desired twist angle, or a combination of these methods.

Polarization rotators can also be designed to utilize both the twistangle and retardation to alter the polarization and ellipticity ofincident light. As an example, consider an input beam of linearlypolarized light with its electric field vector parallel to the directorof a twisted nematic structure. According to the Jones matrix methods(see, for example, “Optics of Liquid Crystal Displays”, by Pochi Yeh andClaire Gu, John Wiley and Sons, 1999), the output light has ellipticityand azimuth orientation given by:$e = {\tan\left( {\frac{1}{2}{\sin^{- 1}\left\lbrack {\frac{\Gamma\phi}{X^{2}}\sin^{2}X} \right\rbrack}} \right)}$${\tan\quad 2\psi} = \frac{2\phi\quad X\quad\tan\quad X}{{\left( {\phi^{2} - \frac{\Gamma^{2}}{4}} \right)\tan^{2}X} - X^{2}}$where ψ is the angle of the major axis of the polarization ellipsemeasured from the local director axis at the exit plane. Here, φ is thetwist angle of the TN structure, Γ is the phase retardation as definedabove, and: $X = {\sqrt{\phi + \left( \frac{\Gamma}{2} \right)^{2}}.}$For example, for 550 nm light, a polarization rotator having abirefringence of 0.12, a thickness of 1.62 μm, and a twist angle of 64°can alter the polarization of linearly polarized light to light withellipticity of −1.

The polarization rotator can be formed using one or more differentlayers (e.g., coatings) of material. For example, multiple layers ofmaterial can be deposited on a particular substrate or polarizer withoptional solvent removal steps and, optionally, partial or full curingbetween deposition of the layers. This can be particularly useful if theparticular substrate or polarizer is sensitive to temperature, humidity,or both. Multiple applications of material can reduce the temperature ortime needed to drive away the solvent or cure the material. As anotherexample, layers of material for the polarization rotator can be formedon different substrates or polarizers and then the two layers broughttogether. This provides a method for combining (e.g., laminating)individual components into a single article. Optionally, an annealingstep at elevated temperature can be performed to facilitate diffusion,coupling, or alignment between two or more layers of polarizationrotator material.

A liquid crystal material can be selected which includes reactivefunctional groups that can be used to crosslink the material.Alternatively, a crosslinking or vitrifying agent can be included withthe liquid crystal material in the composition used to form thepolarization rotator. The liquid crystal material can be aligned asdesired (for example, in a nematic, twisted nematic, or chiral nematicphase) and then crosslinked or otherwise vitrified to retain thealignment. Such crosslinking can be performed by a variety of processesincluding, for example, by photoinitiated, electron-beam, or thermalcuring.

Other materials can be included in the polarization rotator or thecomposition used to form the polarization rotator. For example, adiffusing or scattering material can be included to cause the diffusionor scattering of light, if desired, by the polarization rotator. Asanother example, an absorbing material can be included to absorb lightof a particular wavelength if, for example, a colored appearance or theremoval of a colored appearance is desired. Examples of suitableabsorbing materials include, for example, dyes and pigments. In someembodiments, a dichroic dye material (e.g., a material thatpreferentially absorbs light of one polarization) is used. Inparticular, a dichroic dye material can be desirable if the dichroic dyematerial is capable of being aligned within the polarization rotator.Suitable dichroic dye materials include, for example, iodine, as well asanthraquinone, azo, diazo, triazo, tetraazo, pentaazo, and mericyaninedyes, Congo Red (sodium diphenyl-bis-α-naphthylamine sulfonate),methylene blue, stilbene dye (Color Index (CI) =620),1,1′-diethyl-2,2′-cyanine chloride (CI =374 (orange) or CI =518 (blue)),2-phenylazothiazole, 2-phenylazobenzthiazole,4,4′-bis(arylazo)stilbenes, perylene compounds,4-8-dihydroxyanthraquinones optionally with 2-phenyl or 2-methoxyphenylsubstituents, 4,8-diamino-1,5-naphthaquinone dyes, and polyester dyessuch as Palanil™ blue BGS and BG (BASF AG, Ludwigshafen, Germany). Theproperties of these dyes, and methods of making them, are described inE.H. Land, Colloid Chemistry (1946), incorporated herein by reference.Still other dichroic dyes, and methods of making them, are discussed inthe Kirk Othmer Encyclopedia of Chemical Technology, Vol. 8, pp. 652-661(4th Ed. 1993), and in the references cited therein, all of which areincorporated herein by reference.

Other additives include, for example, oils, plasticizers, antioxidants,antiozonants, UW stabilizers, curing agents, and crosslinking agents.These additives can be reactive with the liquid crystal material ornon-reactive.

In one embodiment, a polarization rotator/polarizer is formed using atwisted nematic structure of a liquid crystal material that alsoincludes absorbing molecules that are oriented with the liquid crystalmaterial. In one example, the absorbing molecules align with thedirection of the liquid crystal material. Light having a polarizationparallel to the director of the liquid crystal material is absorbed andlight having a polarization perpendicular to the liquid crystal materialis transmitted. This embodiment of a polarization rotator also acts as apolarizer. This particular polarization rotator can be, for example, a“clean-up” polarizer positioned after a reflective polarizer to enhancethe extinction of light of the unwanted polarization state.

The optical properties, including indices of refraction, of any materialused in the polarization rotator can be wavelength dependent. Forexample, a thickness corresponding to a half wave retarder for onewavelength may generate less than a half wave retardation for a secondwavelength. In at least some embodiments, particularly displayapplications, it is desirable to reduce or minimize variation over awavelength range, for example, over the visible spectrum of light (e.g.,wavelengths from about 380 to about 800 nm). One method of reducing thewavelength dependence (i.e., decreasing the chromaticity) of thepolarization rotator includes the formation of two or more separatelayers using different materials and aligning the two layers so that theoptical axes of the layers are crossed at a particular angle. Forexample, the optical axes of the layers can be crossed at 90° to eachother. The materials are selected to obtain a polarization rotator inwhich Δnd/λis substantially constant (e.g., varying by no more than 10%or 5%) for a desired wavelength range. For example, a layer ofpolypropylene can be laid crosswise over a layer of polycarbonate (orvice versa) to obtain an element with substantially uniform opticalretardation over the entire range of visible light wavelengths.Preferably, the difference between the wavelength dependence of theoptical distance through the layer for the two films is substantiallyuniform over the wavelength range of interest. The relative thickness ofeach of the films can be adjusted to modify the wavelength dependence ofthe composite of the films.

Alignment layers can optionally be used with the polarization rotator todefine the optical axis at surfaces of the polarization rotator. Thisoptical axis can be at an angle parallel to the surface of the alignmentlayer. In addition, in at least some instances, a tilt angle away fromthe surface of the alignment layer can be defined by the alignmentlayers. Alignment layers are particularly useful with liquid crystalmaterials to define the alignment of the director of the liquid crystalat the surfaces of the polarization rotator. Alignment layers can beprovided at opposing surfaces of the liquid crystal material (e.g., apolarization rotator). One alternative includes using a single alignmentlayer and relying on the pitch and thickness of the polarization rotatorto determine the alignment at the opposing surface.

Alignment layers can be separately formed layers or can be part of oneor more of the other optical components of the film. For example, thepolarizer can also act as an alignment layer. Optionally, the liquidcrystal material can be crosslinked after alignment to maintain thealignment. Optionally, one or more of the alignment layers can beremoved from the device after crosslinking or vitrifying the LCmaterial.

A variety of methods are known for the preparation of alignment layersbecause alignment layers have been used in other components including inLC cells. Generally, one group of known techniques for making alignmentlayers involves mechanical or physical alignment, and a second groupinvolves chemical and photoalignment techniques.

One commonly used mechanical method of making an alignment layerincludes rubbing a polymer layer (e.g., poly(vinyl alcohol) orpolyimide) in the desired alignment direction. Another physical methodincludes stretching or otherwise orienting a polymer film, such as apoly(vinyl alcohol) film, in the alignment direction. Any number oforiented polymer films exhibit alignment characteristics for LCmaterials, including polyolefins (such as polypropylenes), polyesters(such as polyethylene terephthalate and polyethylene naphthalate), andpolystyrenes (such as atactic-, isotactic-, orsyndiotactic-polystyrene). The polymer can be a homopolymer or acopolymer and can be a mixture of two or more polymers. The polymer filmacting as an alignment layer can include one or more layers. Optionally,the oriented polymer film acting as an alignment layer can include acontinuous phase and a dispersed phase. Yet another physical methodincludes obliquely sputtering a material, such as SiO_(x), TiO₂, MgF₂,ZnO₂, Au, and Al, onto a surface in the alignment direction. Anothermechanical method involves the use of microgrooved surfaces, such asthat described in U.S. Pat. Nos. 4,521,080, 5,946,064, and 6,153,272,all of which are incorporated herein by reference.

An alignment layer can also be formed photochemically. Photo-orientablepolymers can be formed into alignment layers by irradiation ofanisotropically absorbing molecules disposed in a medium or on asubstrate with light (e.g., ultraviolet light) that is linearlypolarized in the desired alignment direction (or in some instancesperpendicular to the desire alignment direction), as described, forexample, in U.S. Pat. Nos. 4,974,941, 5,032,009, and 5,958,293, all ofwhich are incorporated by reference. Suitable photo-orientable polymersinclude polyimides, for example polyimides comprising substituted1,4-benzenediamines.

Another class of photoalignment materials, which are typically polymers,can be used to form alignment layers. These polymers selectively reactin the presence of polarized ultraviolet light along or perpendicular tothe direction of the electric field vector of the polarized ultravioletlight, which once reacted, have been shown to align LC materials.Examples of these materials are described in U.S. Pat. Nos. 5,389,698,5,602,661, and 5,838,407, all of which are incorporated herein byreference. Suitable photopolymerizable materials include polyvinylcinnamate and other polymers such as those disclosed in U.S. Pat. Nos.5,389,698, 5,602,661, and 5,838,407. Photoisomerizable compounds, suchas azobenzene derivatives are also suitable for photoalignment, asdescribed in U.S. Pat. Nos. 6,001,277 and 6,061,113, both of which areincorporated herein by reference.

Additionally, some lyotropic liquid crystal materials can also be usedas alignment layers. Such materials, when shear-coated onto a substrate,strongly align thermotropic LC materials. Examples of suitable materialsare described in, for example, U.S. patent application Ser. No.09/708,752, incorporated herein by reference.

As an alternative to alignment layers, the liquid crystal material ofthe polarization rotator can be aligned using an electric or magneticfield. Yet another method of aligning the liquid crystal material isthrough shear or elongational flow fields, such as in a coating orextrusion process. The liquid crystal material may then be crosslinkedor vitrified to maintain that alignment. Alternatively, coating theliquid crystal material on an aligned substrate, such as orientedpolyesters like polyethylene terephthalate or polyethylene naphthalate,can also provide alignment.

The polarization rotator 106 or depolarizing layer 112 can be formedusing any material or construction, preferably suitable to web-basedprocessing, that is capable of affecting the polarization state oflight. The depolarizing layer 112 can be a diffuse layer, such as adiffuse adhesive layer, that somewhat scrambles the polarization. Thedepolarizing layer 112 may be an elliptical depolarizer using pre-tilt.Roughened, structured, or prismatic surfaces can also be used as thedepolarizing layer 112. The depolarizing layer 112 may include a polymermatrix containing particles that deflect light. The depolarizing layer112 preferably does not absorb light within the desired depolarizationwavelength range to any significant degree (e.g., absorbance is no morethan about 1%, 5%, or 10%).

The optical properties of the depolarizing layer 112 are determined bythe selection or manipulation of various parameters, including theoptical indices of particles, the size and shape of the particles, thevolume fraction of the particles, and the thickness of the depolarizinglayer 112.

The magnitude of the index match or mismatch along a particular axisbetween the material (e.g., particles and polymer matrix) of thedepolarizing layer will directly affect the degree of scattering oflight polarized along that axis. In general, scattering power varies asthe square of the index mismatch. Thus, the larger the index mismatchalong a particular axis, the stronger the scattering of light polarizedalong that axis. Conversely, when the mismatch along a particular axisis small, light polarized along that axis is scattered to a lesserextent and is thereby transmitted specularly through the volume of thebody.

The size of the particles can have a significant effect on scattering.If the particles are too small (e.g., less than about 1/30 thewavelength of light in the medium of interest) and if there are manyparticles per cubic wavelength, the depolarizing layer behaves as amedium with an effective index of refraction somewhat between theindices of the two phases along any given axis. In such a case, verylittle light is scattered. If the particles are too large, the light isspecularly reflected from the particle surface, with very littlediffusion into other directions. When the particles are too large in atleast two orthogonal directions, undesirable iridescence effects canalso occur. Practical limits may also be reached when particles becomelarge in that the thickness of the depolarizing layer becomes greaterand desirable mechanical properties are compromised.

The shape of the particles can also have an effect on the scattering oflight. The depolarizing factors of the particles for the electric fieldcan reduce or enhance the amount of scattering in a given direction. Theeffect can either add or detract from the amount of scattering from theindex mismatch.

The shape of the particles can also influence the degree of diffusion oflight scattered from the particles. This shape effect is generally smallbut increases as the aspect ratio of the geometrical cross-section ofthe particle in the plane perpendicular to the direction of incidence ofthe light increases and as the particles get relatively larger.

One method of forming the depolarizing layer is as a film by dispersingfine particles in a resin. The fine particles composed of polymer beads,etc. are fixed in the resin (for example, a polymer matrix), of whichthe refractive index is different from that of the fine particles. Therefractive index and the size of the fine particles are in the range offrom 1.0 to 1.9 and from 1 to 10 μm, respectively. For example, theresin may be a photosensitive acrylic resin having a refractive index of1.5. The material obtained by dispersing the fine particles in thephotosensitive resin is applied to the first substrate and thensubjected to ultraviolet radiation while being pressed. The opticalscattering layer which has a planar surface is thereby formed.

The absorbing polarizer 102, reflective polarizer 104, or secondreflecting polarizer 108 may have a variety of constructions. Reflectivepolarizers can take a variety of forms. Suitable reflective polarizersinclude those which have two or more different materials of differingrefractive index in alternating layers or as a dispersed phase within acontinuous phase. Polymeric multilayer reflective polarizers aredescribed in, for example, U.S. Pat. Nos. 5,882,774 and 5,965,247 andPCT Publication Nos. W095/17303; W095/17691; W095/17692; W095/17699;W096/19347; and W099/36262, all of which are incorporated herein byreference. One commercially available form of a multilayer reflectivepolarizer is marketed as Dual Brightness Enhanced Film (DBEF) by 3M, St.Paul, Minn. Inorganic multilayer reflective polarizers are described in,for example, H. A. Macleod, Thin-Film Optical Filters, 2nd Ed.,Macmillan Publishing Co. (1986) and A. Thelan, Design of OpticalInterference Filters, McGraw-Hill, Inc. (1989), both of which areincorporated herein by reference. Diffuse reflective polarizers includethe continuous/disperse phase reflective polarizers described in U.S.Pat. No. 5,825,543, incorporated herein by reference, as well as thediffusely reflecting multilayer polarizers described in U.S. Pat. No.5,867,316, incorporated herein by reference. Other reflective polarizersare described in U.S. Pat. Nos. 5,751,388 and 5,940,211, both of whichare incorporated herein by reference.

Another example of a reflective polarizer is formed using cholestericliquid crystal material. The cholesteric liquid crystal polarizertransmits right-or left-handed circularly polarized light at awavelength corresponding to the optical length of the pitch of thecholesteric liquid crystal. The light that is not transmitted isreflected and is circularly polarized in the opposite helicity.Cholesteric liquid crystal reflective polarizers are described in, forexample, U.S. Pat. Nos. 5,793,456, 5,506,704, 5,691,789, and EuropeanPatent Application Publication No. EP 940 705, all of which areincorporated herein by reference. As the LCD requires the input oflinearly polarized light, cholesteric reflective polarizers aretypically provided with a quarter wave retarder to convert thetransmitted circularly polarized light into linearly polarized light.Suitable cholesteric reflective polarizers are marketed under thetradename TRANSMAX™ by Merck and Company, Incorporated and NIPOCS™ byNitto Denko Corporation.

Another type of polarizer is an absorbing polarizer. These polarizersare typically made of a material that is oriented and absorbs light of aparticular polarization. Examples of such polarizers include orientedpolymer layers that are stained with a dichroic dye material, such asiodine or metal chelates. Examples of such constructions include astretched poly(vinyl alcohol) layer that is stained with iodine. Adiscussion of suitable absorbing polarizers can be found in, forexample, U.S. Pat. Nos. 4,166,871, 4,133,775, 4,591,512, and 6,096,375,which are all herein incorporated by reference.

Another type of absorbing polarizer includes an oriented polymer,optionally made without additional dyes or stains, which includessegments, blocks, or grafts of polymeric material that selectivelyabsorb light. One example of absorbing polarizer made without stains ordyes is an oriented copolymer that includes poly(vinyl alcohol) andpolyvinylene blocks, where the polyvinylene blocks are formed bymolecular dehydration of poly(vinyl alcohol). A discussion of polarizersmade without dyes or stains can be found in, for example, U.S. Pat. No.3,914,017 and 5,666,223, both of which are herein incorporated byreference.

Oriented polymer films of the above-described absorbing polarizers canalso act as an alignment layer for the polarization rotator, if desired.In one embodiment, an oriented poly(vinyl alcohol) absorbing polarizeris provided over a reflective polarizer (see, for example, U.S. Pat. No.6,096,375). The oriented poly(vinyl alcohol) absorbing polarizeroptionally acts as an alignment layer for a polarization rotator formedusing liquid crystal material disposed on the absorbing polarizer.

As indicated above, in place of the polarizer (element 202 asillustrated in FIG. 3), another polarization-altering element can beused. Such polarization-altering elements include, for example,compensation films. These films alter the polarization of light toprovide a different elliptical or circular polarization. This canprovide a wider horizontal viewing angle, vertical viewing angle, orboth for a display.

The film can have more than one polarizer or other polarization-alteringelement. For example, a polarization rotator can be disposed between twopolarizers. Moreover, the film can include more than one polarizationrotator. In addition, other optical components can be included in thefilm, including, for example, microstructured prism films (such asdescribed in, for example, U.S. Pat. Nos. 5,932,626 and 6,044,196, bothof which are incorporated herein by reference), diffusion layers,scattering layers, and selective wavelength absorbing and transmittinglayers. Other layers can be incorporated into the film which do notsubstantially alter the optical properties of the article including, forexample, adhesive layers and substrates.

A variety of different articles can be constructed. These articles canbe constructed in a number of different ways. In addition to the methodsdescribed herein, additional examples of methods of making the articlesand methods of making polarization rotators and examples of polarizationrotators are described in the copending U.S. patent application Ser. No.09/965417, entitled “Methods of Making Polarization Rotators andArticles Containing the Polarization Rotators”, Docket No. 56233US002,filed on Sep. 27, 2001 and incorporated herein by reference. Inparticular, any of the individual elements of the article can begenerated separately, sequentially, or simultaneously. For example, twoor more of the elements (e.g., a reflective polarizer and thepolarization rotator or depolarizing layer) can be coextruded or can besimultaneously coated onto an optionally removable substrate. As anotherexample, an element (e.g., the polarization rotator) can be coated orotherwise disposed onto a previously formed layer (e.g., a polarizer).Alternatively, the individual elements can be formed separately andlaminated together. A film can be formed using any combination of thesemethods. For example, a reflective polarizer and a polarization rotatorcan be coextruded; a reflective polarizer can be coated onto apolarization rotator; or a reflective polarizer can be laminated to apolarization rotator to form the article.

The elements of the article can be integrated together to form thearticle by a variety of methods which will typically depend on factorssuch as, for example, the types of layers to be integrated together, themethod of forming the individual elements, and the materials of theelements. It will be understood that several different methods can beused for a single film (e.g., one polarizer and the polarization rotatormay be coextruded and then another polarizer laminated thereon). Methodsof integrating elements include, for example, coextrusion, coating,adhesive lamination, heat lamination, diffusion at elevatedtemperatures, reactive coupling between reactive groups on the twolayers, and crosslinking. When an adhesive is used, the adhesive ispreferably optically transparent over the wavelength range of interest,unless the adhesive is also used as an optical layer within the film.

The transflective assembly 100 can be used in a variety of applicationsincluding an LCD and other electronic displays, including cell phone andPDA (personal digital assistants) applications. One preferredapplication is a WNI (white non-inverting) transflector. FIG. 8illustrates one embodiment of an LCD. It will be recognized that otherLCD configurations are known and that the transflective assembly 100 canbe used in those display configurations. The configuration of FIG. 8 isprovided as an example to illustrate the use of the transflectiveassembly 100. An LCD 400 includes an LC cell 402, an optional polarizer410, an absorbing polarizer 102, a reflective polarizer 104, apolarization rotator 106 or a depolarizing layer 112 or both, a secondreflective polarizer 108, an analyzer 406, a backlight 408, and areflector 412. FIG. 9 illustrates another embodiment of an LCD. The LCD400 includes an LC cell 402, an optional polarizer 410, a single filmtransflective assembly 100, an analyzer 406, a backlight 408, and areflector 412. The transflective assembly 100 includes an absorbingpolarizer 102, a reflective polarizer 104, a polarization rotator 106 ordepolarizing layer 112, and a second reflective polarizer 108.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

1. A display comprising a liquid crystal cell; a backlighting systempositioned to provide light to the liquid crystal cell; and atransflective assembly positioned between the backlighting system andthe liquid crystal cell, the transflective assembly comprising apolarization rotating layer positioned between a first reflectivepolarizer and a second reflective polarizer.
 2. The display of claim 1,wherein the first and second reflective polarizers have paralleltransmission axes.
 3. The display of claim 1, wherein the first andsecond reflective polarizers have different polarization axes.
 4. Thedisplay of claim 1, wherein the polarization rotating layer comprises apolarizing element.
 5. The display of claim 1, wherein the polarizationrotating layer comprises a compensation film.
 6. The display of claim 1,wherein the transflective assembly further comprises a light diffusingelement disposed between the first and second reflective polarizers. 7.A display comprising a liquid crystal cell; a backlighting systempositioned to provide light to the liquid crystal cell; and atransflective assembly positioned between the backlighting system andthe liquid crystal cell, the transflective assembly comprising adepolarizing layer positioned between a first reflective polarizer and asecond reflective polarizer.
 8. The display of claim 7, wherein thefirst and second reflective polarizers have parallel transmission axes.9. The display of claim 7, wherein the first and second reflectivepolarizers have different polarization axes.